A defibrillator is a medical device for defibrillation and cardioversion. It can be targeted by power surges, and cardiac arrhythmias such as ventricular fibrillation (defibrillation) or ventricular tachycardia, atrial portable defibrillator fibrillation and atrial flutter (cardioversion end). Defibrillators will be ready in intensive care in emergency rooms, many other places in the hospital and in vehicles of the emergency services and many doctors' offices since the 1990s increasingly also in many public buildings such as railway stations, airports and other locations for use by medical laymen. Also, there are already offers devices for private homes, since they can be operated by voice instructions by any person and such emergencies occur mostly at home.

In 85 percent of all sudden cardiac death at first exists a so-called ventricular fibrillation. A defibrillator can interrupt this circular electrical stimulation in the heart by simultaneous stimulation of at least 70 percent of all cardiac muscle cells. In a large number of cells simultaneously depolarized, which means that these cells (a relatively long time about 250 ms = refractory period of the cells) portable defibrillator are no longer excitable. The rotating shaft is virtually cut off the road and my heart is back in a state in which can take the natural conduction stimulation of the heart again. Essential for them to defibrillation is the earliest possible deployment, as the ventricular fibrillation caused by the undersupply of the brain can lead to oxygen (Gehirnischämie) within a short time to massive neurological deficits. For this reason, be placed in the public space increasingly automated external defibrillators (AED). The successful use of an AED stands or falls with the correct implementation of the cardio-pulmonary resuscitation. The AED is only a complement to, not substitutes.